1. Field of the Invention
This invention relates to containers for storage and transportation of spent nuclear fuel.
2. Description of the Prior Art
Spent nuclear fuel (SNF) discharged from reactors emits highly piercing gamma and neutron radiation and thermal energy. SNF is typically stored in deep pools filled with water to dissipate heat and to attenuate the gamma and neutron radiation generated by the fuel. An alternative to storing the SNF in water filled pools (wet storage) consists of the so-called dry storage concept wherein the SNF is stored in a configuration inside a heavy-walled vessel referred to as the "cask." Dry storage systems usually consist of two basic constituents, a multi-purpose canister (MPC) and an overpack. The term MPC refers to a fuel basket within an enclosure vessel. After SNF is placed in a fuel basket, the fuel basked is placed in an enclosure vessel, and the resultant MPC, which is the "fuel confinement system," is then placed into a neutron/gamma isolation device called an "overpack." The MPC within the overpack is referred to as the "dry storage system." There are two types of overpack: storage overpacks and transportation overpacks. The transportation overpack can be used for transportation as well as storage, and is therefore dual purpose.
Lehnert, et al., U.S. Pat. No. 5,438,597 assigned to Vectra Technologies, Inc., discloses one MPC in which spent nuclear fuel rods are stored in substantially square shaped sleeves which are maintained in spaced apart axial alignment relative to one another by a plurality of generally circular plates, having a plurality of generally square shaped apertures formed therethrough. The plates are maintained in spaced apart axial alignment relative to one another by eight elongate rectangular plates. The Vectra spacer disk technology fuel basket is stored in a concrete horizontal storage module designed to withstand all normal and abnormal condition loads, such as earthquakes, tornadoes, flooding and other natural phenomena, as well as complete loss of ventilation. The Vectra MPC is designed to be transportable, but the overpack is not.
MPCs have also been proposed by Hinderer, et al., U.S. Pat. No. 5,406,601 assigned to Babcock & Wilcox Company, in which the basket is formed from multiple layers of rowed plates that cooperate with the cask body to provide the required radiation shielding, thermal, and structural requirements. The plates have complementary shapes and partial hex grooves machined therein, such that complete channels for the fuel cells are formed when the plates are mated for insertion into the cask body. The plates have narrowed diameter sections and are held together by bands around the circumference of the plates at these sections.
Efferding, U.S. Pat. No. 4,800,283 assigned to Westinghouse Electric Corp., disclosed a basket structure having a cell assembly of square cross-sectioned tubes formed from two sets of parallel plates which are slotted approximately one-half the distance of their lengths and interfitted in "egg crate" fashion to define an array of square, elongated cells which are welded along their entire lengths at every intersection in order to rigidify the structure. Disposed in each of the cells defined by the interlocking plates is an elongated container having a square cross-section. The outside walls of each of the elongated containers is clad with neutron-absorbing material such as Boron carbide encased in aluminum. Mounting brackets in the corners of each of the cells serve to mount and uniformly space each of the containers from the interior walls of its respective cell. Former plates of the basket structure having a circular outer edge serve as shock absorbing members which circumscribe the cell assembly and maintain the cell assembly within the cylindrical vessel of the transportation cask. The transportation cask is preferably made of carbon steel and the circular former plates are preferably made of aluminum, which expands when subjected to the heat of the fuel rods and thereby expand within the cylindrical vessel. In fact, the cell assembly and former plates of the basket structure are formed from the same type of aluminum alloy, i.e., aluminum 6061-T45, for reasons which are set forth in the patent.
Sierra Nuclear discloses a dry storage system using a storage-only concrete storage cask, and a dual purpose steel welded basket for both transport and storage of spent nuclear fuel rods. The fuel basket is a welded assembly fabricated from square steel tubes. Structural support in the horizontal direction is provided by curved, horizontal spacer disks located along the length of the basket assembly. The fuel basket is placed in a concrete cask which is accomplished by employing a transfer cask or shielding bell to move a loaded fuel basket to the concrete cask.
In spite of the many attempts by other research and design teams, no one has proposed a multi-purpose canister or fuel basket having the capability to store multiple fuel assemblies and provide axial as well as vertical support, have the capability to transport the heat generated by the fuel assemblies to its outside surface, be sufficiently rugged to enable convenient handling, and capable of withstanding handling accidents.
Furthermore, no one has provided a concrete overpack structure which effectively insulates the outside environment from the gamma and neutron radiation while effectively and efficiently rejecting heat to the atmosphere, while at the same time being sufficiently rugged to withstand an accidental tipover event, as well as earthquakes, tornadoes, and other extreme environmental events.
The thermal problem in a cask consists of rejecting the decay heat produced by the spent nuclear fuel such that the temperature of the fuel cladding remains below the threshold value at which long-term temperature effects would not degrade the cladding. For a given ambient temperature, the temperature of the fuel cladding would rise as the resistance in the path of heat transmission is increased. Recognizing that the resistance to conductive heat transfer offered by a 1-inch gap filled with air is equal to nearly 2,000 inches of steel, it is quite apparent that introducing even a small gap (say, 0.25 inch), would greatly magnify the thermal resistance offered by an overpack wall 12 inches thick. In other words, the gaps exacerbate the thermal problem in cask design in a most direct and significant manner.
In the pre-MPC era, the cask designer was able to persuade himself that the basket-to-cask gap would be closed or greatly diminished by a tight manufacturing tolerance and by the thermal growth of the basket during operation. The only location where a designer had to contend with an undiminished physical gap was the spacing between the stored fuel and the storage cell wall.
The assumed absence of a serious gap barrier elsewhere in the cask enabled the designers to utilize the so-called "box-and-disk" design illustrated in the Lenhert patent, supra. In this design, each storage cell is defined by a box; an array of boxes is arranged in a square grid pattern maintained by a number of transverse disks. Square holes cut in the disks provide lateral support to the boxes; an array of disks reduce the unsupported longitudinal span of the boxes and provide the path of heat transmission to the overpack. The designers often utilized a strong, but poor, heat conductor alloy material such as SA240-304 S/S, and highly conductive, but weak, aluminum disks in an alternating pattern to respectively provide structural support and heat transfer path. The Lehnert patent shows a typical box-and-disk construction.
In NAC International's NAC-STC fuel basket, the box is made from a thin (18 gauge) sheet metal stock. Panels of Boron carbide encased in aluminum are laid on the four sides of the box and secured in place by four 0.019 inch-thick sheathing panels. Welding the sheathing to the box to create a sealed chamber for Boron carbide encased in aluminum leads to excessive heat induced deformation and warpage of the box. For boxes made of low conductivity materials such as austenitic stainless steel, severe welding induced warpage is simply unavoidable.
Disks, made from austenitic stainless steel, would experience deformation as well if the square holes are made by burning and grinding. Heat induced warpage of the disks can be mitigated, however, if the holes are made by broaching.
STC's designers seem to have recognized the potential of warpage of the fabricated composite box and sought to remedy the fit-up problem by making relatively large square holes in the disks.
In summary, the box/disk construction forces the designer to introduce gaps which are most detrimental to the transmission of heat. The manufacturing mandated box-to-disk gaps are the essence of the "thermal problem" in this design. For the MPC construction to be viable, we have found it is necessary that gaps within the basket not essential to its operation, and needed only to enable manufacturing assembly, be eliminated. Otherwise, the heat duty rating of the cask will be severely limited.
Some of the structural frailties of the box and disk design become clear when one considers that the deformation of the boxes and disks in the manufacturing process makes the condition of support at the box-hole interface largely uncertain. A most inimical situation would develop if the deformation of the disk ribs and the box led to a knife edge contact condition, since under such condition the disk may shear the thin (0.019 inches thick) sheathing under a horizontal drop event. Even under the ideal conditions of support, the inherent structural limitations of the box and disk design is easily understood when a horizontal drop event is considered.
Calculations show that stress levels in the boxes for a horizontal drop event typical of transport conditions are too high and raise the concern that under a horizontal drop event, the boxes will bulge, reducing the flux trap gap between them with direct consequence to the package's nuclear criticality characteristics.
Prior closure systems for MPCs use multiple circulator disk-shaped lids, the first being thick to provide radiation shielding for subsequent welding operations. The first lid is welded in place if there are two lids, or not welded in place if there are three lids. The second lid provides an additional seal or in the case of three lids provides spacing and shielding. After each lid is installed, welding machines are set up and the weld must be inspected, causing the operator to receive radiation exposure. Each additional lid requires set up time for the welding machine.